21148 Ibeneme S.I et al./ Elixir Geoscience 67 (2014) 21148-21153

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Geoscience

Elixir Geoscience 67 (2014) 21148-21153

Hydrogeophysical Approach in Aquifer-Trend Determination around the Western Part of the Lower Imo River Basin Southeastern Ibeneme S.I and Selemo A.O.I Geosciences Department, Federal University of Technology, Owerri, , Nigeria.

ARTICLE INFO ABSTRACT Article history: This geophysical investigation of groundwater is aimed at delineating the aquiferous units Received: 29 December 2013; around the western part of the Lower Imo River Basin by determining their depths, Received in revised form: thicknesses, resistivities and the potential borehole depth at various locations within the 18 January 2014; area employing the technique of Vertical Electrical Sounding (VES) using the Accepted: 31 January 2014; Schlumberger array. Twenty two sounding stations were established. Four to six geo- electric layers comprising the top soil, clayey sand, dry sandstone, saturated sandstone, Keywords shaley sand and sandy shale were delineated with the later usually occurring as the last Benin Formation, layer. The third and fourth layers underlying dry sandstone form the aquiferous unit. This Groundwater, unit was found to have an average resistivity value range of 10.7–6060 Ωm and an average Imo River, thickness of 32 m. Longitudinal Conductance increases towards the Southeast with a high Schlumberger, closure around Umuduru Egbe Aguru. Conversely, Transverse Resistance increases Transverse Resistance. towards the Northwest with a very high closure around Dikenafai and Mgbee axis. These confirm the difficulties often experienced during ground water exploitation in these localities. The Northern part of the study area has high thickness of aquifer units ranging from 50-80m. Similarly too, high depth to water table and consequent high possible Total Drill Depth (TDD) were mapped within the Northern part of the area. The reverse is the case for the South being dominated by the highly prolific Benin Formation. It was advised that care ought to be taken in drilling and casing at shallow aquiferous areas to maintain proper sanitary condition so as to reduce the risk of groundwater contamination. © 2014 Elixir All rights reserved.

Introduction engineering investigations, environmental studies among others Water is essential for life. It had been and will continue to (Kearey et al (2002), Keller and Frischknecht, (1966), Khalil be a hot topic in both the political and scientific arena for years (2009), Telford et al (1970)) . Vertical Electrical Sounding, VES, to come (Miller, 2006). The importance of groundwater as a using Schlumberger array has gained wide applications owing to supply source to the socio-economic development of a country its cost effectiveness and easy data acquisition and is tremendous. Groundwater is water contained within the open interpretation, among numerous advantages, compared to other spaces between soil, sand, gravel and within fractures in rock. resistivity measurements (Ibeneme et al 2013b). This method The water comes from rain and melting snow which soaks has been a veritable tool to mapping aquifer in all geologic through the ground and seeps into formations called aquifers. It terrains in the country and beyond. A number of authors have usually moves slowly from high places towards low places and recently attempted the prediction of aquifer hydraulic properties ultimately discharges into a nearby surface water body. The fact in Nigeria from surface electrical resistivity soundings (Onuoha remains that some 74% of the earth consists of water. But 99.4% and Mbazi, 1988; Mbonu et al., 1991; Ekwe, et al., 2006; of this water cannot be used because it’s either saline or locked Igboekwe et al., 2006; Onu and Ibezim,2004; among others). up in glaciers or ice sheets. Less than 0.01% is present in rivers Various hydrogeophysical and hydrogeeological works have and lakes. The remaining water is present in soils and rocks as been carried out in areas close to the study area by numerous groundwater (Adagunodo, 2011). In Nigeria, Federal, state and workers. These include Uma and Egboka, 1985, Opara, 1997, local Government including individuals and politicians are busy Adishi, 1994, Ibe and Uzoukwu, 2004 Uma, 1989. Uma and drilling boreholes with little or no consideration to Egboka, 1985 worked on the water resources of Owerri and its hydrogeology, fluid mechanics and its relationship with the host environs. They studied critically the aquifer systems of the area rocks and lithologic Formations. It has been observed that most and result showed that the unconsolidated sands and their drillers tend to drill boreholes in areas that are being recharged interfingering clay underlying the area gave rise to their aquifer by lacerates landfills, polluted surface Rivers, even at times in systems. The first and second aquifer layers are unconfined and old cemeteries/ graveyards and abandoned sucker pits and semi-confined respectively. The second having high field sewage systems. There is a serious lapse in groundwater capacity. They showed that water table is less than 20m from the development in Nigeria with a lot of abortive boreholes being ground surface in the South but ranges from 25-35m deep drilled. Drilling boreholes is now, all-comers-affairs. Therefore Northwards. They also verify the aquifer parameters such as there is need for proper studies of underground formations. hydraulic conductivity K, transmissivity T, and Storativity S, the Since the development of electrical resistivity surveying method results showed that hydraulic conductivity ranges between 0.72 in 1900’s, it has been applied to solve problems associated with x 10 2 cms -1 to 24 x 10 2 cms -1. The corresponding transmissivities groundwater exploration, mapping fractures and cavities, range between 1.37m2s -1 to 7.40m2 -1. Adishi, 1994 Carried out

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studies on the weathering layer parameters of Ihiagwa, Owerri November and December and ends between March and April using resistivity and Seismic refraction methods. He estimated (Iloeje 1991). The rainfall distribution consists of two minima the average thickness and average velocity of the weathered and two maxima. The first minima are in November and layer. Also he deduced its electrical resistivity. Opara, 1997 December while the second minimum is in August which is worked on the aquifer systems of the using usually associated with August break. From February, total resistivity and geophysical well logging methods. He affirmed rainfall increases sharply to primary maxima in June and July. the multilayered aquifer system and estimated the average depth The second maximum is in September which increases sharply to the saturated water level as 6m and this decreased coastwards. and subsequently decreases in November and December The aim of the study is to use the electrical resistivity sounding (NIMET 2012). The area comprises of true rainforest, methods to define the geometry of the aquifers in the study area characterized by an abundance of plant species. A storeyed and delineate sites for drilling of productive boreholes and sequence of canopies may be found in most part of the forest. estimate aquifer hydraulic parameters with a view to mapping the different aquifer systems in the area and evaluating their trend. The Study Area The area lies within latitudes 5o38’N to 5o50’N and longitudes 7o00’E to 7o24’E. It is located towards the northern part of Imo State. The area is bounded to the northwest by Anambra State, to the northeast by Okigwe and to the south by Owerri. The main rivers that traverse the study area are , Orashi and Ogidi Rivers which are tributaries to the Imo River. The terrain of the study area is characterized by two types of landforms viz: high undulating ridge and flat topography. These undulating ridges and flat lands are somewhat related to the bedrock underlying the area. Borehole lithologic logs reveal that the undulating Hills and ridges are underlain by a succession of thick unconsolidated sandstones and relatively thin clay units belonging to the Benin Formation.

Figure 2. Geologic Map of the Study Area

Figure 3. Elevation map of the area in meters above sea level Geology of the area The area is part of the Benin Formation (Figure 2). The Figure 1. Location Map of the Study Area showing VES geology of the Benin Formation has been given by Short and points Stauble 1967, Reyment 1965, Hospers 1965 among others. The The ridges trend in the north-south direction and have Benin Formation is made up of friable sands and minor elevation ranging from about 90-240m above sea level (Figure intercalation of clay. The sand units are mostly coarse grained, 3). In between these ridges and valleys of the , pebbly, poorly sorted and contain lenses of fine grained sands Njaba River and their tributaries further south, the ridges give (Short and Stauble, 1967, Onyeagocha, 1980). Benin Formation way to clearly flat topography. The flat southern part is is one of the three recognized subsurface stratigraphic units in underlain by thick unconsolidated sand and minor clay lenses the modern Niger Delta. It is the fresh water bearing massive and stringers. The area has two seasons, namely rainy and dry continental sand and gravel deposited in an upper deltaic seasons. The mean annual rainfall is between 200m and 2250m environment. It was formerly designated as coastal plain sands and the annual rainfall is usually heavy. On a monthly basis, the (Reyment, 1965). As the study component in most areas form rainfall amount at any location is not uniform but exhibits a more than 90% of the sequence of the layers, permeability, marked seasonality. The date of the onset of the rainy season is transmissivity and storage coefficient are high. usually between February and March and ends around October The Formation starts as a thin edge at its contact with and November, whereas the onset of the dry season is between Ogwashi/Asaba Formation in the north of the area and thickens 21150 Ibeneme S.I et al./ Elixir Geoscience 67 (2014) 21148-21153

seawards. The Benin Formation conformably overlies the Field Procedure Ogwashi/Asaba Formation. The Ogwashi/Asaba Formation The vertical electrical sounding (VES) method using the (Lignite series) is also predominantly sandy with minor clay Schlumberger electrode array was applied in this study. Current units. The Formation is characterized by lignite seams at various and potential electrodes made of stainless steel were used. The levels. The lignite Formation has thickness of about 300m. This current electrodes were expanded to a maximum of 250-300m, is underlain by the Ameki Formation with thickness of 1460m, while the potential electrodes are kept fixed and are only moved which is in turn underlain by Imo shale and Nsukka Formations in order to get a reasonable value of the potential gradient. The successively. soundings were conducted with conventional current electrode Table 1. Stratigraphic succession in the area (adapted from reels, a multicore potential cable and switching unit. Current and Uma and Egboka 1985). potential electrodes were made of non-polarizable electrodes, Tertiary AGE FORMATION LITHOLOGY the current electrode spacing was increased in logarithmic steps Miocene- Benin Medium-coarse grained while the potential electrodes were kept constant and were only Recent Formation poorly consolidated varied to obtain a satisfactory reading to the potential drop. A sands with clay lenses total of 22 soundings were carried out in the study area. The data and stringers. in form of earth’s resistance measurement were converted to Oligocene- Ogwashi Consolidated sands with apparent resistivity value by multiplying it with the Miocene Asaba lignite seams at various Schlumberger geometric factor as shown in equation 1 layers. 2 Eocene Ameki Clayey sandstone and ρa = (a /b – b/4) R …………………(1) Formation sandy claystones where Paleocene Imo shale Laminated clayey shale ρa = Apparent resistivity in Ohm-m Nsukka Sandstones intercalated a = AB/2, the Half Current Electrode Separation in meters Formation with shales and coal beds b = MN, Potential electrode separation in meters Data acquisition and instrumentation R = Instrument reading in Ohms A total of twenty two (22) VES were carried out. The G is the Geometric Constant which is a function of the maximum spread was 250-300m for AB/2 (m) and the data were electrode configuration employed during the survey. The data acquired under favourable weather condition. A variety of collected in the field were presented by plotting the apparent electrodes arrangement is possible for resistivity measurement in resistivity ( ρa) values against the electrode spacing (AB/2) on bi- the field, but Schlumberger array has been found to be log graph. Quantitative interpretation of the VES curves convenient and reliable in most terrains. The theory behind the involved partial curve matching and computer iteration use of this method has been fully documented (Ibeneme et al technique employing the method of Zohdy (1976) . As a good 2013a). The instrument used for this study is the ABEM SAS fit, (up to 95% correlation) was obtained between field and 4000. This consists of a basic unit called the Terrameter SAS model curves, interpretation was considered as right. 4000 which can be supplemented as desired with SAS 3000C Results and discussion Booster and SAS log 200. SAS, which stands for Signal The results of the twenty two (22) Vertical Electrical Averaging System, is a method whereby consecutive readings Sounding (VES) are presented in Table 2 while their are taken automatically and the results are averaged interpretation and subsequent analysis to suite the content of this continuously. The continuously update running average is work are as presented hereunder. The Resistivity of the presented automatically on the display unit. For this study an delineated aquiferous zones in the study area varies from 10.7- average of four readings were taken at each point. The 6060 Ωm. Low resistive aquifer units were observed towards the tetrameter SAS 4000 can be operated in two modes: the eastern part of the study area (Figure 4). These areas resistivity surveying mode and the voltage measuring mode. In geologically correspond to areas occupied by the Imo Shale the resistivity surveying mode, it comprises of a battery Formation. Aquifers encountered in these areas are mainly powered, deep penetration resistivity meter with an output perched aquifers entrapped within the sandstone member of the sufficient for a current electrode separation under good Imo Shale Formation (Umuna Sandstone). surveying condition. In the voltage measuring mode, the SAS comprises of self potential instrument that measures natural D.C potentials, and displays the result in millivolts (mV). The overall range extends from 0.01mv to 500v. Non-polarizable electrodes are available for self potential surveys. The tetrameter SAS 4000 consists of three main units housed in a single casing: the transmitter, the receiver and the microprocessor. The electrically isolated transmitter sends out well defined and regulated signal currents. The receiver discriminates noise and measures voltage correlated with transmitted signal current (resistivity survey mode) and also measures uncorrelated D.C potentials with the same discrimination and noise filtering (voltage measuring mode). The microprocessor monitors and controls operation and calculates results. In geological survey the equipment permits natural and induced signals to be measured at extremely low levels with excellent penetration and lower consumption. Moreover it can be used in a wide variety of application where effective signal/noise discrimination is needed. Figure 4. Variation of Aquifer Resistivity in the study area

21151 Ibeneme S.I et al./ Elixir Geoscience 67 (2014) 21148-21153

closure around Umuduru Egbe Aguru. Conversely, Transverse Resistance increases towards the Northwest with a very high closure around Dikenafai and Mgbee axis. These confirm the difficulties often experienced during ground water exploitation in these localities. The Northern part of the study area has high thickness of aquifer units ranging from 50-80m (Figure 7). Similarly too, high depth to water table (Figure 8) and consequent high possible Total Drill Depth (TDD) (Figure 9) were mapped within the Northern part of the area. The reverse is the case for the South being dominated by the highly prolific Benin Formation.

Figure 5. Variation of Longitudinal Conductance in the study area

Figure 8. 3-D map of Depth to water table variation in the study area

Figure 6. Variation of Transverse Resistance in the study area

Figure 9. 3-D map of possible Total Drill Depth (TDD) in the study area Conclusion And Recommendation The western parts of the Lower Imo River Basin have good aquifer systems. It was observed that the shallow aquifer systems were obtained towards the south characterized by the Figure 7. 3-D map of the Aquifer thickness distribution in dominant Benin Formation whereas the deeper aquifer systems the study area were observed to occur in the areas underlain by the Ameki and The Longitudinal Conductance and Transverse Resistance Imo Shale Formations. Low resistive aquifer units were vary in opposite directions (Figures 5 and 6). Longitudinal observed towards the eastern part of the study area. Conductance increases towards the Southeast with a high 21152 Ibeneme S.I et al./ Elixir Geoscience 67 (2014) 21148-21153 Table 2. Summary of the VES results indicating real location names and other hydraulic parameters VES Location Coordinates Elevation Depth Aquifer Aquifer No of Possible Aquifer Longitudinal Transverse to Resistivity thickness layers total Material conductance resistance Latitude Longitude Ft m water drill table depth (m) (m) 1 Anara I 5o42.081’N 7O10.2118’E 671 204.5 21.6 111.00 23 5 60 Sand 0.059 49506 2 Anara II 5o42.125’N 7O10.2118’E 601 183.2 19.3 1120.0 15.9 6 55 Sandstone 0.058 39424 3 Anara III 5o42.125’N 7O10.049’E 651 198.4 26.0 1010.2 19 6 60 Sand 0.073 45955 4 Dump Site 5o42.416’N 7O10.562’E 558 170.1 18.1 3190.0 51.9 4 85 Sandstone 0.028 226171 Ugwu Orlu 5 Umudara 5o42.811’N 7O10.540’E 407 124.1 18.3 3190.0 51.9 4 85 Sandstone 0.028 2915.4 mgbee (Don’s House) Orlu 6 Dikenafai 5o42.729’N 7O10.846’E 445 135.6 39.9 86.0 15.6 5 50 Sandy shale 0.527 233632 Ideato South 7 Umuokwara 5o42.911’N 7O10.837’E 891 271.6 26.17 2980.0 38.5 4 90 Sandstone 0.064 86856 Akokwa, Ideato North 8 Uloano 5o42.853’N 7O10.173’E 426 129.8 36.1 2310.0 10.9 6 50 Sandstone 0.030 977920 Ndugba Isu L.G.A 9 Chief 5o42.243’N 7O02.285’E 434 132.3 63.1 6060.0 49.9 5 128 Sandstone 0.050 684780 Marcus House 10 Orodo 5o42.151’N 7O01.567’E 457 139.3 20.0 539.0 12.6 4 47 Sandstone 0.090 17571.4 Mbaitolu L.G.A 11 Agbaja, 5o38.010’N 7O20.361’E 389 118.6 63.0 237.0 53 4 125 Sand/Sandstone 1.089 274.92 Isiala Mbano 12 Umuna 5o38.001’N 7O20.361’E 360 109.7 50.4 1200.0 24.3 4 47 Sand/Sandstone 0.312 89640 Onumo 13 Okohia 5o49.972’N 7O49.972’E 510 155.4 72.9 260.0 37.1 4 120 Sandy shale 1.222 10375.2 Umuchiri I 14 Okohia 5o48.673’N 7O48.673’E 571 174.0 22.4 60.0 16.9 4 50 Sandy shale 2.465 2381.58 Umuchiri II 15 Okpuala 5o40.784’N 7O40.784’E 312 95.0 20.7 10.7 31.2 4 63 Sandy shale 5.023 555.33 Aboh 16 Umuduru 5o42.681’N 7O19.631’E 401 122.2 82.0 5070.0 42 4 133 Sandy shale 0.126 628680 Egbe Aguru 17 Ndiamazu 5o49.972’N 7O12.113’E 542 165.2 95.0 940.0 53 4 138 Shaly sand 0.193 120320 Arondizuogu 18 Arondizuogu 5o49.992’N 7O11.932’E 602 183.4 56.3 2340.0 36.6 4 105 Sandstone 0.053 217386 19 Okai umuna 5o43.921’N 7O20.002’E 372 113.4 52.2 2650.0 54.8 4 115 Sandstone 0.064 283550 20 Umuezeala 5o44.231’N 7O20.242’E 307 93.6 58.9 1010.0 71.1 4 140 Sandstone 0.155 131300 Umuna 21 Umuezeala 5o44.230’N 7O19.922’E 310 94.5 45.8 1900.0 81.2 4 135 Sandstone 0.112 241300 Ezifuoke Umuna 22 Aforezihu 5o44.230’N 7O18.021’E 301 92.0 39.5 30.5 61.5 4 112 Sandy shale 7.228 3080.5 Umuna

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